Power devices operated in integrated circuits typically operate with a voltage in the range 20V to 1.2 kV and typically higher than 30V or 50V or so. Power devices typically operate with a current in the range 10 mA to 50 A and typically higher than 0.1 A and smaller than 5 A. Such devices may also be referred to as “high voltage/power devices”. These devices are typically capable of delivering from a few mWatts to 1 Watt or even a few tens of Watts of power. Their application may range from domestic appliances, electric cars, motor control, and power supplies to RF and microwave circuits and telecommunication systems.
Lateral devices in integrated circuits have the high voltage/low voltage main terminals (variously called the anode/cathode, drain/source and emitter/collector) and the control terminal (termed the gate or base) placed at the top surface of the device in order to be easily accessible. In addition the back surface is usually electrically connected via a metal enriched epoxy and a lead frame, usually made of copper, to ground. This is referred to as the back terminal. The epoxy is used as a package die attach and is enriched with particles of metal (e.g., silver) to increase its thermal conductivity and allow good thermal dissipation of heat from the silicon device to the package. In power ICs, such devices are often monolithically integrated with CMOS-type or BiCMOS-type low voltage and/or low power circuits and therefore it is desirable that the lateral high voltage devices are CMOS compatible. It is also possible that several high voltage, power devices are integrated within the same chip. (It will be appreciated that terms such as “top” and “bottom”, “above” and “below”, “lateral” and “vertical”, “beneath”, and “under” and “over” may be used in this specification by convention and that no particular physical orientation of the device as a whole is implied).
MOS bipolar power devices, such as the lateral insulated gate bipolar transistor (LIGBT) shown in FIG. 1, are based on MOS control with bipolar current conduction in the drift layer and the lowly-doped substrate underneath. Such devices are based on the conductivity modulation concept. At high levels of charge injection, when the current in the device increases, a mobile charge of electrons and of holes is built up in the drift layer, and deep into the substrate region leading to a desirably sharp increase in the conductivity of the drift layer. The mobile charge accumulated, known as plasma, in the on-state dictates the on-state/switching performance of the device given that the plasma must be removed in order to switch the device to the off-state. The plasma level is one to three orders of magnitude higher than the doping level, depending on the current density and the lifetime of the carriers.
Van der Pol et al., Microelectronics Reliability, Volume 39, Issues 6-7, Pages 863-868 (June-July 1999) describes medium power, complementary bipolar devices that have no drift region and whose active transistors are vertically-oriented. Furthermore, a lower, substrate region of the IC does not form a part of the active device—the devices are actually isolated from such lower regions of the IC.
There remains a need for an LIGBT having improved characteristics, for example proper functioning, lower losses and/or faster switching over a wider range of operating conditions (e.g., any combination of one or more predetermined range of continuous and/or switching current between main terminals, voltage between main terminals, junction and/or ambient temperature, etc.).
For use in understanding the present invention, the following disclosures are referred to:                U.S. Pat. No. 7,381,606 (corresponding to application U.S. Ser. No. 11/783,966, which is related to application U.S. Ser. No. 11/133,455 (U.S. Pat. No. 7,301,220)), F. Udrea, Cambridge Semiconductor Ltd., published Mar. 20, 2008;        WO-A-02/25700, Udrea, Cambridge Semiconductor Ltd., published 2006 Mar. 2;        U.S. Pat. No. 6,703,684, Udrea, Cambridge Semiconductor Ltd., published Apr. 11, 2002;        US-A-2004-0084752, Udrea, Cambridge Semiconductor Ltd., published May 6, 2004;        US-A-2004-0087065, Udrea, Cambridge Semiconductor Ltd., published May 6, 2004;        Microelectronics Reliability Vol. 39, Issues 6-7, June-July 1999, Pages 863-868, European Symposium on Reliability of Electron Devices, Failure Physics and Analysis, J. A. van der Pol et al.        